These connecting devices equip in particular military airplanes such as certain fighters or troop and/or hardware transporters, as well as helicopters, which, in order to successfully complete their mission, must be refueled during their flight.
Refueling is performed from a tanker airplane on which is provided at least one connecting device comprising a refueling pipe or the like in fluidic communication with at least one relevant tank of the tanker airplane and which, after linkage with the connecting device of the airplane to be refueled, provides for the transfer of fuel from the tank or tanks of the tanker airplane to the tank or tanks of the refueled airplane.
Of course, the tanker airplane may itself be equipped with a connecting device so as, in its turn, to be refueled in flight from another tanker airplane.
The connecting devices currently used onboard refueled aircraft generally comprise:                a fixed connecting boom, which is hollow and able to cooperate via its distal end with the refueling device of the tanker airplane; and        a support structure, fixed to the structure of the refueled aircraft, in particular to the top front part of the fuselage in its longitudinal plane of symmetry, said support structure serving as base fixed to said boom and being provided with an internal passage making it possible to join the proximal end of the connecting boom to a tank to be filled of said refueled aircraft.        
Having regard to the loads engendered on the connecting boom when it is linked to the pipe of the refueling device and during the transfer of the fuel, the support structure which takes up and absorbs the loads (forces and moments) transmitted by the boom, is designed rigidly.
According to a first known embodiment, the support structure comprises an ax symmetric body with internal passage and three supports carrying the body. More particularly, to the front face of the body is joined the proximal end of the connecting boom, while the rear face of said body is joined to fluidic piping to convey the fuel flowing through the internal passage of the body, to the tank. The three supports distributed along the body are, on the one hand, fixed to the latter and, on the other hand, rest externally on the skin of the fuselage while also being fixed to transverse structural frames of the fuselage, which are situated internally with respect to the skin, by way of fixing members.
The three supports then constitute built-in links embedded vis-à-vis the frames aimed at transmitting the loads in the frames, while the axisymmetric body opposes the bending moment imposed by the boom.
According to a second likewise known embodiment, the support structure of the boom is also composed of three supports fixed to the structural frames of the fuselage by way of the skin and joined fixedly together by plates so as to define a structure of the box type which is very rigid and whose interior forms the internal passage. The connecting boom is fixed, via its proximal end, to the front support of the box structure. This embodiment makes it possible to lessen the intensity of the stresses in the built-in embedding with the structure of the airplane, but has the drawback of adding a rigid and quasi-undeformable node to the fuselage.
Although they are widely used, these connecting devices nevertheless have drawbacks, chiefly by reason of their rigidity.
Specifically, if these built-in solutions make it possible to efficiently take up the loads exerted on the connecting boom and transmitted by the latter during linkage thereof to the tanker airplane's refueling device, on the other hand they disturb the surrounding structure of the refueled airplane, which cannot deploy in an optimal manner since all the movements apt to occur are disabled in proximity to the built-in embedding. Consequently, as the structure of the fuselage is designed to be able to “breathe” during flight following pressure variations between the interior of the airplane and the exterior environment, the prevention of these movements through the rigidity of the “support structure-fuselage structure” link may lead to the appearance of cracks in them. A solution then consists in strengthening the zone of the relevant structure so as to decrease the level of the stresses, but it makes the whole assembly heavier and even further rigidities said zone, so that the overdimensioned structure absorbs more load and new cracks appear.
For example, the structural frames have a C cross section to resist pressure. Thus, the web of each frame, perpendicular to the skin of the fuselage, works in shear and its lower flange or heel makes it possible to avoid the warping or distortion of the web, its upper flange being fixed to the respective support by fixings and the skin of the fuselage. If the radial loads introduced by the boom into the support structure are taken up well by the web of each support, on the other hand, the axial load imposes a secondary moment on the frame which tends to bring about the warping of the web. A strengthener is then adjoined to the web to rigidify each support but this solution then brings about an increase in the weight and the increase in the rigidification brings about the appearance of cracks.